-LRB- CNN -RRB- -- Is al Qaeda looking to ground transportation as a promising target after the introduction of intense security measures for air travel ? And are its supporters able to operate out of Iran without hindrance ?

Those two questions have been raised after the announcement by Canadian police on Monday that they had thwarted a plot to attack a rail link between Canada and the United States .

The Canadian announcement has puzzled some terrorism experts , because Iranian authorities are widely seen as hostile to al Qaeda . Iran has denied the conspiracy could have been plotted from its territory .

Canadian officials say they did not think the alleged conspirators were `` state-sponsored '' but suspect al Qaeda elements in Iran had provided two men in Canada with direction , guidance and information .

As CNN National Security analyst Peter Bergen noted , `` If these allegations are true , it would appear to be the first time that al Qaeda elements based in Iran have directed some kind of plot in the West . ''

While Western intelligence agencies have long thought that al Qaeda operatives are present on Iranian soil , they were thought to be under some form of house arrest and under strict control .

Many al Qaeda sympathizers and members took refuge in Iran in the aftermath of the post 9/11 invasion of Afghanistan , including one son of Osama bin Laden , Saad bin Laden , and a son-in-law , Sulaiman Abu Ghaith . Abu Ghaith left Iran in February this year and entered Turkey , but was subsequently arrested in Jordan and flown to the United States in March .

It 's still unknown whether he left Iran of his own volition or was told to go . But Iran has become increasingly hostile to al Qaeda as it has become a Sunni sectarian group , especially in Iraq and Syria , as much as an anti-western outfit .

In Syria , the al Qaeda-linked Jabhat al Nusra is one of the most effective fighting groups against the regime of Bashar al-Assad , which is heavily backed by Tehran .

The vast majority of Iranians are from the Shia sect of Islam , and al Qaeda has frequently targeted Shia communities and mosques in Iraq . Al Qaeda leader Ayman al Zawahiri has often railed against Iran in his speeches .

But the United States says there appears to have been some tactical co-operation between the Islamic Republic and al Qaeda . In 2011 , the U.S. Treasury Department accused Iran of a `` secret deal with al-Qa ` ida allowing it to funnel funds and operatives through its territory . ''

Railroad Targets

Another aspect of the alleged Canadian plot is that it underlines the enduring interest of al Qaeda and its supporters in attacking railroad systems , seeing such operations as relatively cheap and easy to carry out -- with potentially devastating results .

A senior law enforcement official in the United States told CNN 's Gloria Borger that one of the routes targeted was between New York and Toronto , with the alleged conspirators looking at possible targets in the Toronto area , such as trestles over roads or water .

`` I worry about the vulnerability of mass transit because we know they 've been on al Qaeda 's radar screen , '' Robert Liscouski , a former director of infrastructure protection at the Department of Homeland Security , told CNN .

Liscouski said that while significant steps had been taken in the United States to make air travel more secure , not enough was being done to reduce the vulnerability of mass transit to bomb attacks .

He told CNN that there needed to be greater investment in random searches by agents with explosive trace detection equipment and greater deployment of CCTV cameras , saying the extra investment needed in mass transit security amounted to a fraction of the economic cost of a successful attack .

Al Qaeda 's interest in attacking railroads was underlined in the wake of the raid that killed Osama bin Laden in May 2011 . Documents recovered from his compound in Abbottabad in Pakistan revealed that early in 2010 , al Qaeda had discussed an idea to derail a train in the United States , several U.S. officials told CNN . The plan was to be executed coinciding with the 10th anniversary of the September 11 attacks .

By then , two plots against U.S. railroad targets had already been foiled . In March 2008 , American al Qaeda recruit Bryant Neal Vinas discussed a plan to attack the Long Island Railroad with al Qaeda operatives in a jihadist encampment in the tribal areas of Pakistan . Vinas later testified that his idea was for an al Qaeda operative to leave a suitcase bomb with a timer on a train and then exit at a station before the device exploded .

His al Qaeda handler then decided the best scheme would be to launch a suicide bombing on a train as it entered a tunnel , to create maximum devastation .

In September 2009 , the FBI broke up an al Qaeda plot by Najibullah Zazi and two others to bomb New York subway lines . At their subsequent trial two of the group testified that they settled on a plan to target New York subways during rush hour so they could cause the heaviest number of casualties , instill fear , and inflict maximum economic damage .

Authorities were alerted to the planned attack only days before Zazi drove from Denver to New York , after his communication with a known al Qaeda facilitator in Pakistan was intercepted . Zazi had apparently been asking his handler for help in making the explosive TATP .

Al Qaeda cells have also plotted attacks on railway systems in Spain and Germany . The most lethal was carried out on board several commuter trains in Madrid in March 2004 , in which 191 people were killed and more than 1,700 injured .

There is little evidence to link `` al Qaeda central '' with the Madrid attack , which involved a cell of mainly Moroccan immigrants . Several weeks later the cell that carried out the attack attempted to blow up a bridge on the Madrid-Seville high-speed train line .

The plot was abandoned after the terrorists realized they had been spotted by nearby workers , who alerted police , a Spanish official told CNN . The official added that hundreds could have been killed given the quantity of explosives left at the scene .

The explosives were similar to those used in the Madrid attacks . Bomb disposal experts found between 24 pounds -LRB- 10 and 12kg -RRB- of explosives connected to a detonator with a 440-foot -LRB- 135-meter -RRB- cable .

On July 7 , 2005 , al Qaeda suicide bombers killed dozens on the London underground system . A follow-on attempt two weeks later failed when the bombs malfunctioned .

In December 2012 , German police discovered a crude improvised explosive device placed in a bag on a platform at the main railway station in Bonn . German authorities have stated they have `` robust evidence '' Islamist extremists were involved but no arrests have yet been made .

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Canadian authorities suspect al Qaeda elements in Iran provided guidance to alleged plotters

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Iran has denied the alleged rail conspiracy could have been plotted from its territory

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Western intelligence has long thought any al Qaeda operatives in Iran are under strict control

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Al Qaeda 's interest in attacking railroads underscored following death of Osama bin Laden